As is well known, a hydraulically-actuated electronically-controlled unit injector fuel system has a plurality of injectors, each of which, when actuated, meters a quantity of fuel into a combustion chamber in the cylinder head of the engine. Actuation of each injector is accomplished through valving of high pressure hydraulic fluid within the injector under the control of the vehicle's microprocessor based electronic control module (ECM).
Generally, sensors on the vehicle impart engine information to the ECM which develops actuator signals controlling a solenoid on the injector and the flow of hydraulic fluid to the injector. The solenoid actuates pressure balanced poppet valves (as shown in the '867 and '829 patents) in the injector to port high pressure fluid to an intensifier piston which causes injection of the fuel at very high pressures. The pressure at which the injector injects the fuel is a function of the hydraulic fluid flow supplied the injector by a high pressure pump while the timing of the injector is controlled by the solenoid. Both functions are controlled by the ECM to cause precise pulse metering of the fuel at desired air/fuel ratios to meet emission standards and achieve desired engine performance. Tightening emission standards and a demand for better engine performance have resulted in continued refinement of the control techniques for the injector. Generally the pump flow output has to be variable throughout the operating range of the engine. For example, one manufacturer may desire a constant pump flow throughout an operating engine speed range except at the higher operating engine speeds whereat the injectors are valving so quickly reduced pump flow may be desired even though more fuel is being injected by the injectors to the combustion chambers. Other manufacturers may desire to rapidly change pump flow at any given instant for emission control purposes. For example, the ECM may sense a step load change on the engine and impose a change in the fuel/air ratio to overcome the effects of a transient emission.
High pressure hydraulic actuating fluid is supplied to each injector by a high pressure pump in fluid communication with each injector through a manifold/rail fluid passage arrangement. As noted in the '867 patent, the high pressure pump is either a fixed displacement, axial piston pump or alternatively a variable displacement, axial piston pump. If a fixed displacement pump is used, a rail pressure control valve is required to variably control the pressure in the manifold rail by bleeding a portion of the flow from the high pressure pump to a return line connected to the engine's sump. For example, the '867 patent mentions varying the output of the high pressure pump by the rail pressure control valve to pressures between 300 to 3,000 psi. A variable displacement pump can eliminate the rail control valve if the flow output of the variable pump can timely meet the response demands imposed by the HEUI system.
A fixed displacement, high pressure pump is typically used in HEUI systems because of cost considerations. The pump is sized to match the system it is applied to. It is well known that the flow of a fixed displacement pump increases, generally linearly, with speed. Accordingly, the fixed displacement pump is sized to meet HEUI system demands at a minimal engine speed which is less than the normal operating speed ranges of the engine. Higher engine speeds produce excess pump flow which is dumped by the rail pressure control valve to return. The excess flow represents an unnecessary power or parasitic drain on the engine which the engine manufacturers have continuously tried to reduce.
For example, the '111 patent shows a control scheme in which excess pump flow is passed to an idle injector but at a rate insufficient to actuate the injector. The system is stated to allow elimination of the rail pressure control value and permit a more accurate sizing of the fixed displacement pump. However, the system does not avoid unnecessary parasitic engine power drains imposed by the pump. The pump must still be sized to produce a set flow sufficient to actuate the injectors at a low speed and that flow increases with pump speed.
The prior art has also developed variable output pumps for use in a HEUI system as disclosed, for example, in the '829 patent. Variable output, axial piston pumps do not have the reliability of a fixed displacement, axial piston pump and are more expensive. More significantly, the response time demands for pump output flow in a HEUI system is becoming increasingly quicker and a variable pump may be unable to change output flow within the time constraints of a HEUI system unless a rail pressure control valve is used.